lecture 22

Lecture 22Lecture 22

The Death of StarsThe Death of Stars

RIP

AnnouncementsAnnouncements

Tonight is the last regular Lab. A Tonight is the last regular Lab. A signup sheet will be posted next signup sheet will be posted next to the door for the make-up lab to the door for the make-up lab next week. Please indicate which next week. Please indicate which labs you are missing so that I can labs you are missing so that I can decide how to do the make-up.decide how to do the make-up.

The Main SequenceThe Main Sequence

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On the HR diagram, the sun starts here.

Early Red GiantEarly Red Giant

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By the time the sun first becomes a red giant, it is now here on the diagram (in the region for giants).

Just Before The Helium Just Before The Helium FlashFlash

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By the time the sun reaches the helium flash, it is here on the diagram.

The Red Giant BranchThe Red Giant Branch

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This path stars follow as they become red giants is often called the giant branch of the HR diagram.

A Helium-Burning StarA Helium-Burning Star

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After the helium flash, the sun becomes, smaller, warmer, and dimmer than before.

The Horizontal BranchThe Horizontal Branch

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Once a solar-type star begins helium burning, it ends up somewhere along this horizontal line on the HR diagram.


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For this reason, helium-burning solar-type stars are called horizontal branch stars.


The core helium burning phase is The core helium burning phase is sometimes called “sometimes called “the second the second main sequencemain sequence” because of ” because of similarities to the hydrogen similarities to the hydrogen burning phase:burning phase:– Energy is again produced in the core Energy is again produced in the core

(but using a different fuel).(but using a different fuel).– Pressure-Temperature thermostat is Pressure-Temperature thermostat is

very effective again: star’s very effective again: star’s size/temperature stays very stable.size/temperature stays very stable.

The End Of The The End Of The ReprieveReprieve Important Differences:Important Differences:

– Helium burning doesn’t last as long.Helium burning doesn’t last as long.– Helium fusion is not as efficient as Helium fusion is not as efficient as

hydrogen fusion: hydrogen fusion: produces less produces less energy per kg of nuclear fuelenergy per kg of nuclear fuel..

– Sun is still 40 times brighter than Sun is still 40 times brighter than today.today.

– Starts to run out of helium in only Starts to run out of helium in only about 250 million years.about 250 million years.

The Second AscensionThe Second Ascension

As helium fuel runs out:As helium fuel runs out:– Carbon core starts Carbon core starts

shrinking.shrinking.– Helium burning Helium burning

begins in shell around begins in shell around carbon core.carbon core.

– Hydrogen burning Hydrogen burning begins in shell around begins in shell around helium shell.helium shell.

The star is swelling into a The star is swelling into a red giant again! Called red giant again! Called the the second ascensionsecond ascension..

About How Big Will About How Big Will Our Sun Get?Our Sun Get? This phase is the largest and This phase is the largest and

brightest our sun will ever get.brightest our sun will ever get.

L = 4,800

T = 3,000 K

R = 260

Here’s original size for comparison.

The Death Of Earth?The Death Of Earth?

During this During this phase, the sun phase, the sun will swallow the will swallow the Earth.Earth.

Probably won’t Probably won’t make it out to make it out to Mars.Mars.

The Second Giant The Second Giant BranchBranch

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There are two giant branches on the HR diagram, side by side.

The Second Giant The Second Giant BranchBranch

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The second one is called the asymptotic giant branch.

So stars in their second ascension are often called AGB stars.

AGB GiantsAGB Giants

Very large, Very large, luminous, and red.luminous, and red.– R > 200R > 200– L = 5,000-10,000L = 5,000-10,000– T ~ 3,000 KT ~ 3,000 K

Energy source is Energy source is helium and helium and hydrogen shell hydrogen shell fusion.fusion.

Star has inert C, N, Star has inert C, N, O core.O core.

AGB GiantsAGB Giants

AGB Giants AGB Giants experience experience significant mass loss.significant mass loss.– Gravity too low to hold Gravity too low to hold

onto distended outer onto distended outer layers.layers.

– Dust forms in cool Dust forms in cool outer layers; “reflects” outer layers; “reflects” core light, helping to core light, helping to push outer layers out push outer layers out into space.into space.

– Lose up to 1 solar Lose up to 1 solar mass every 100,000 mass every 100,000 years.years.

Mass Loss In Giant Mass Loss In Giant StarsStars Giant stars have Giant stars have

strong stellar strong stellar winds and weak winds and weak surface gravity.surface gravity.

During the giant During the giant phase, these phase, these winds carry off a winds carry off a large percentage large percentage of the star’s mass.of the star’s mass.

By The End Of The By The End Of The Giant Phase…Giant Phase…

Up to half or Up to half or more of the star’s more of the star’s gasses can end gasses can end up as a nebula up as a nebula around the giant around the giant star.star.

The Planetary NebulaThe Planetary Nebula

Forms in two Forms in two stages:stages:1.1. Early in AGB stage, Early in AGB stage,

mass loss occurs in mass loss occurs in the form of a slow the form of a slow cool wind. Forms cool wind. Forms an expanding shell an expanding shell of gas around the of gas around the star.star.

2.2. After expulsion of After expulsion of outer layers, core is outer layers, core is exposed to space.exposed to space.

The Planetary NebulaThe Planetary Nebula

Hot, fast stellar Hot, fast stellar wind from core wind from core slams into cool slams into cool expanding shell.expanding shell.

Gas glows by Gas glows by emission. Result emission. Result is a is a planetary planetary nebulanebula..

Planetary NebulaePlanetary Nebulae

Mass loss not Mass loss not necessarily necessarily symmetric:symmetric:

Cold shell may be Cold shell may be less dense at poles.less dense at poles.– Easier for hot wind Easier for hot wind

to get through the to get through the poles.poles.

– Results in an Results in an asymmetric nebula asymmetric nebula (like an hourglass).(like an hourglass).

Planetary NebulaePlanetary Nebulae

Planetary Nebulae Planetary Nebulae are very short are very short lived:lived:– Expansion of Expansion of

nebula rapidly cools nebula rapidly cools gases.gases.

– Emission fades, Emission fades, nebula becomes nebula becomes too dim to observe too dim to observe after a few after a few 10,000’s of years.10,000’s of years.

The Final CollapseThe Final Collapse

Core finishes Core finishes consuming all nuclear consuming all nuclear fuel.fuel.

Gravity wins!Gravity wins!– Core collapses until Core collapses until

electron degeneracy electron degeneracy prevents further prevents further contraction.contraction.

– What’s left of star is What’s left of star is now about the size of now about the size of Earth, but very, very Earth, but very, very hot: a hot: a white dwarfwhite dwarf star. star.

White Dwarf StarsWhite Dwarf Stars

No nuclear fusion. No nuclear fusion. Star is “dead.”Star is “dead.”

Electron degeneracyElectron degeneracy (From Quantum (From Quantum Mechanics) provides Mechanics) provides the pressure that the pressure that prevents gravity from prevents gravity from collapsing the star.collapsing the star.– Pauli Exclusion Principle: Pauli Exclusion Principle:

No two electrons can No two electrons can be in the same place at be in the same place at the same time, doing the same time, doing the same thing.the same thing.

Electrons can exert a Electrons can exert a powerful outward powerful outward pressure to keep from pressure to keep from getting too close getting too close together!together!

White Dwarf StarsWhite Dwarf Stars

Heat is left over from Heat is left over from energy released energy released during gravitational during gravitational collapse.collapse.

Star starts out very Star starts out very hot: 100,000 K!hot: 100,000 K!

No way to replace No way to replace heat radiated into heat radiated into space. Star slowly space. Star slowly cools down over cools down over billions of years.billions of years.

End stage is End stage is black black dwarfdwarf – but none have – but none have formed yet!formed yet!

The Structure Of A The Structure Of A White DwarfWhite Dwarf Mostly a sphere of Mostly a sphere of

C, N, and O that is C, N, and O that is completely electron completely electron degenerate.degenerate.

Atmosphere of Atmosphere of hydrogen and hydrogen and helium.helium.

Carbon center may Carbon center may crystallize to form a crystallize to form a giant diamond!giant diamond!

Daily Quiz 22 – Daily Quiz 22 – Question 1Question 1What prevents gravity from shrinking a What prevents gravity from shrinking a

white dwarf to a smaller size?white dwarf to a smaller size?

A.A. Helium core fusion.Helium core fusion.B.B. Helium shell fusion.Helium shell fusion.C.C. Hydrogen core fusionHydrogen core fusionD.D. Degenerate electrons Degenerate electrons

(electromagnetic force).(electromagnetic force).

White Dwarf SizesWhite Dwarf Sizes

Higher mass results in smaller, Higher mass results in smaller, denser white dwarf.denser white dwarf.

Upper mass limit of 1.44 solar Upper mass limit of 1.44 solar masses.masses.– Called the Called the Chandrasekar limitChandrasekar limit..– Above this mass, gravity overcomes Above this mass, gravity overcomes

electron degeneracy.electron degeneracy.– The white dwarf collapses!The white dwarf collapses!

White Dwarf SizesWhite Dwarf Sizes

Novae!Novae!

Occur in binary Occur in binary systems.systems.– One star is One star is

“normal” (often a “normal” (often a giant or giant or supergiant).supergiant).

– Other star is a Other star is a white dwarf.white dwarf.

Novae!Novae!

Companion star loses Companion star loses mass to the white dwarf.mass to the white dwarf.

Forms an accretion disk Forms an accretion disk that deposits hydrogen that deposits hydrogen onto the dwarf’s surface.onto the dwarf’s surface.

Hydrogen crushed to Hydrogen crushed to degeneracy.degeneracy.

Pressure and Pressure and temperature increase as temperature increase as more hydrogen is added.more hydrogen is added.

““Kindling point” is Kindling point” is reached, and …reached, and …

Novae!Novae!

Surface of dwarf is Surface of dwarf is consumed in a consumed in a thermonuclear thermonuclear explosion!explosion!

Light output jumps to Light output jumps to 10,000’s to 100,000’s of 10,000’s to 100,000’s of times normal!times normal!

Hydrogen layer is Hydrogen layer is ejected from white ejected from white dwarf.dwarf.

White dwarf is not White dwarf is not “damaged”“damaged”– Process begins again.Process begins again.– Most nova recur!Most nova recur!

And Now: Supernovae!And Now: Supernovae!

A much bigger class of stellar A much bigger class of stellar explosion is called a explosion is called a SupernovaSupernova

Supernovae have Supernovae have twotwo types:types: Supernovae classed by spectrum:Supernovae classed by spectrum:

– Type IType I Spectrum shows Spectrum shows nono hydrogen lines. hydrogen lines. Some Type I SN’s just as bright as Type II: called Some Type I SN’s just as bright as Type II: called

Type Ib.Type Ib. Remaining Type I SN’s soar to 4 billion times solar Remaining Type I SN’s soar to 4 billion times solar

luminosity, then fade quickly: called Type Ia.luminosity, then fade quickly: called Type Ia.

– Type IIType II Spectrum shows hydrogen lines.Spectrum shows hydrogen lines. Caused by core collapse in massive star. Caused by core collapse in massive star.

Hydrogen lines from exploding outer layers of star.Hydrogen lines from exploding outer layers of star.

Type Ia SupernovaType Ia Supernova

Some supernova Some supernova are exploding white are exploding white dwarfs.dwarfs.

How do you blow up How do you blow up a white dwarf?a white dwarf?

Start with a star Start with a star system similar to system similar to setup for a nova:setup for a nova:– White dwarf drawing White dwarf drawing

material from material from companion star.companion star.

Blowing Up White Blowing Up White DwarfsDwarfs

BUT white dwarf is BUT white dwarf is very close to very close to Chandrasekar Limit Chandrasekar Limit (1.44 solar masses).(1.44 solar masses).

Matter “stolen” from Matter “stolen” from companion star companion star drives mass above drives mass above Chandrasekar Limit Chandrasekar Limit before a nova can before a nova can occuroccur..

Blowing Up White Blowing Up White DwarfsDwarfs

White dwarf collapses. Internal White dwarf collapses. Internal temperature reaches kindling point for temperature reaches kindling point for Carbon before dwarf reaches neutron Carbon before dwarf reaches neutron degeneracy.degeneracy.

Gas still electron degenerate – no Gas still electron degenerate – no pressure/temperature thermostat:pressure/temperature thermostat:– Runaway fusion – all carbon fused all at once!Runaway fusion – all carbon fused all at once!– Resulting thermonuclear explosion totally Resulting thermonuclear explosion totally

blasts the white dwarf apart! Result is a Type blasts the white dwarf apart! Result is a Type Ia Supernova!Ia Supernova!

Daily Quiz 22 – Daily Quiz 22 – Question 2Question 2

What can happen to the white dwarf in a close What can happen to the white dwarf in a close binary system when it accretes matter from the binary system when it accretes matter from the companion giant star?companion giant star?

A.A. The white dwarf can become a main sequence The white dwarf can become a main sequence star once again.star once again.

B.B. The white dwarf can ignite the new matter and The white dwarf can ignite the new matter and flare up as a nova.flare up as a nova.

C.C. The white dwarf can accrete too much matter The white dwarf can accrete too much matter and detonate as a supernova type Ia.and detonate as a supernova type Ia.

D.D. Either the white dwarf can ignite the new Either the white dwarf can ignite the new matter and flare up as a nova, or the white matter and flare up as a nova, or the white dwarf can accrete too much matter and dwarf can accrete too much matter and detonate as a supernova type Ia.detonate as a supernova type Ia.

And Now: Type Ib and II And Now: Type Ib and II Supernovae!Supernovae!

The Times Listed Are For An The Times Listed Are For An M=25 StarM=25 Star

The SupergiantsThe Supergiants

Core runs low on Core runs low on H fuel. Collapses H fuel. Collapses and ignites He.and ignites He.– He burning He burning

creates C, N, and creates C, N, and O.O.

– Ignites H to He Ignites H to He burning shell burning shell around core.around core.

– Star’s luminosity Star’s luminosity increases. Swells increases. Swells in size.in size.

Countdown to DisasterCountdown to Disaster

After 7 million After 7 million years:years:– H to He fusion in H to He fusion in

core ends. core ends. – He to C, N, O He to C, N, O

fusion in core fusion in core begins.begins.

– H to He burning H to He burning shell forms.shell forms.

– Star becomes Star becomes supergiant.supergiant.


500,000 years later:500,000 years later:– He in core exhausted.He in core exhausted.– Core collapses, heats up to 800 million Core collapses, heats up to 800 million

K.K.– C, N, O burning begins, producing Ne C, N, O burning begins, producing Ne

and Mg.and Mg. 600 years later:600 years later:

– Core C, N, O supply used up.Core C, N, O supply used up.– Core collapses, heats up to 1.5 billion K.Core collapses, heats up to 1.5 billion K.– Ne and Mg burning begins, producing Si.Ne and Mg burning begins, producing Si.


Six months later:Six months later:– Core supply of Ne and Mg used up.Core supply of Ne and Mg used up.– Core collapses, heats to 3 billion K.Core collapses, heats to 3 billion K.– Si fusion begins, producing Fe.Si fusion begins, producing Fe.

Now there’s a problem! Remember, Now there’s a problem! Remember, we we can’tcan’t fuse iron into heavier fuse iron into heavier elements and make energy!elements and make energy!


One day after Silicon fusion begins:One day after Silicon fusion begins:– Si is running low in the core.Si is running low in the core.– Heat/Pressure from Si fusion cannot Heat/Pressure from Si fusion cannot

support Fe core.support Fe core.– Fe core begins to collapse. Core Fe core begins to collapse. Core

heats up.heats up.– Fe Fe cannotcannot be fused into heavy be fused into heavy

elements (and still release energy)!elements (and still release energy)!


Only milliseconds to go:Only milliseconds to go:– Temperature in Fe core soars above 100 Temperature in Fe core soars above 100

billion K!billion K!– Two nuclear reactions can occur at this Two nuclear reactions can occur at this

temperature:temperature: NeutronizationNeutronization – protons and electrons react to – protons and electrons react to

form neutrons.form neutrons. PhotodisintegrationPhotodisintegration – photons hit Fe nuclei and – photons hit Fe nuclei and

shatter them into He nuclei!shatter them into He nuclei!


Both reactions require energy! Both reactions require energy! Core rapidly Core rapidly cools downcools down!!– Loss of heat/pressure speeds up Loss of heat/pressure speeds up

collapse!collapse!– Result is a catastrophic, runaway Result is a catastrophic, runaway

collapse of the Fe core!collapse of the Fe core!

The Fuse is Lit!The Fuse is Lit!

500 km Fe core 500 km Fe core collapses to 10 km collapses to 10 km across.across.– Reaches same density Reaches same density

as nuclear matter.as nuclear matter.– Core collapse stops Core collapse stops

abruptly as core abruptly as core becomes unimaginably becomes unimaginably rigid.rigid.

– Outer layers of star slam Outer layers of star slam into now rigid core at into now rigid core at extreme speeds.extreme speeds.

– Shockwave forms, Shockwave forms, rocketing outward rocketing outward through the star!through the star!

KABOOM!KABOOM!

One hour later:One hour later:– Shockwave Shockwave

erupts through erupts through surface of star.surface of star.

– Everything but Everything but collapsed core collapsed core blasted into blasted into space: star dies space: star dies in a spectacular in a spectacular explosion!explosion!

The SupernovaThe Supernova

Star-destroying Star-destroying explosion called a explosion called a supernovasupernova..– Light output exceeds Light output exceeds

600 million solar.600 million solar.– Extreme heat/energy Extreme heat/energy

in shockwave results in shockwave results in nuclear fusion in in nuclear fusion in outer layers of star.outer layers of star.

– Fusion reactions in Fusion reactions in supernova create supernova create elements heavier than elements heavier than Fe. Fe.

Type Ib SupernovaeType Ib Supernovae Similar to how a type II Similar to how a type II

supernova happens, but without supernova happens, but without hydrogen lines in the spectra.hydrogen lines in the spectra.

How to “get rid” of hydrogen How to “get rid” of hydrogen lines?lines?– Eject hydrogen-rich outer Eject hydrogen-rich outer

layers before core collapse.layers before core collapse.– Example: Wolf-Rayet stars.Example: Wolf-Rayet stars.

>40 solar masses.>40 solar masses. Extremely unstable: violent Extremely unstable: violent

stellar wind eventually stellar wind eventually ejects outer layers of star.ejects outer layers of star.

After core collapse and After core collapse and supernova, very little supernova, very little hydrogen is left in star to hydrogen is left in star to create spectral lines.create spectral lines.

Type Ib SupernovaeType Ib Supernovae Similar to how a type II Similar to how a type II

supernova happens, but supernova happens, but without hydrogen lines in without hydrogen lines in the spectra.the spectra.

How to “get rid” of How to “get rid” of hydrogen lines?hydrogen lines?– Could also strip outer Could also strip outer

layers by being part of a layers by being part of a binary system.binary system.

Daily Quiz 22 – Daily Quiz 22 – Question 3 Question 3 Why can't massive stars generate Why can't massive stars generate

energy from iron fusion?energy from iron fusion?

A.A. The temperature at their centers The temperature at their centers never gets high enough.never gets high enough.

B.B. The density at their centers is too The density at their centers is too low.low.

C.C. Iron fusion consumes energy.Iron fusion consumes energy.D.D. Not enough iron is present.Not enough iron is present.

Observations of Observations of SupernovaeSupernovae

Supernovae can easily be seen in distant Supernovae can easily be seen in distant galaxies.galaxies.

Local Supernovae and Life on Local Supernovae and Life on EarthEarth

Nearby supernovae (< 50 light years) could kill many Nearby supernovae (< 50 light years) could kill many life forms on Earth through gamma radiation and high-life forms on Earth through gamma radiation and high-

energy particles.energy particles.

At this time, no star At this time, no star capable of producing a capable of producing a supernova is < 50 ly supernova is < 50 ly

away.away.

Most massive star Most massive star known (~ 100 solar known (~ 100 solar masses) is ~ 25,000 masses) is ~ 25,000

ly from Earth.ly from Earth.

The Crab Nebula:

Remnant of a supernova observed in

1054

Supernova RemnantsSupernova Remnants

The Cygnus Loop

The Veil Nebula

Cassiopeia A

Optical

X-rays

The Remnant of SN The Remnant of SN 1987A1987A

Most recent nearby Most recent nearby SN was in February SN was in February 1987.1987.

Ring due to SN Ring due to SN ejecta catching up ejecta catching up with pre-SN stellar with pre-SN stellar

wind; also wind; also observable in X-observable in X-

rays.rays.

Daily Quiz 22 – Daily Quiz 22 – Question 4Question 4Which type of star eventually develops Which type of star eventually develops

several concentric zones of active several concentric zones of active shell fusion?shell fusion?

A.A. Low mass stars.Low mass stars.B.B. Medium mass stars.Medium mass stars.C.C. High mass stars.High mass stars.D.D. White dwarfs.White dwarfs.

The Neutron StarThe Neutron Star

Formed from the Formed from the collapsing iron core collapsing iron core of a massive star.of a massive star.

Core collapses until Core collapses until neutron neutron degeneratedegenerate..

Often 1-2 solar Often 1-2 solar masses squeezed masses squeezed into a ball 20 km into a ball 20 km across!across!


A neutron star has A neutron star has an outer crust (2 km an outer crust (2 km thick) made from thick) made from super-dense iron.super-dense iron.

Inside is an ocean of Inside is an ocean of superfluid superfluid neutronsneutrons that form that form whirlpools under the whirlpools under the surface of the star.surface of the star.


What are they like?What are they like?– Extremely hot: Extremely hot:

1 million K1 million K– Rotate very fast: Rotate very fast:

conservation of conservation of angular momentum.angular momentum.

– Extremely powerful Extremely powerful magnetic fields.magnetic fields.

– Extreme surface Extreme surface gravity.gravity.


Very powerful Very powerful magnetic field.magnetic field.

Believed to Believed to create beams of create beams of electromagnetic electromagnetic radiation.radiation.

PulsarsPulsars

As a neutron star rotates, the As a neutron star rotates, the beams sweep through space.beams sweep through space.

PulsarsPulsars

When a beam sweeps over the When a beam sweeps over the Earth, we see a flash of light.Earth, we see a flash of light.

PulsarsPulsars

Since the neutron star rotates so Since the neutron star rotates so quickly, the flashes (“pulses”) of light quickly, the flashes (“pulses”) of light happen many times a second.happen many times a second.

When observed with telescopes, these When observed with telescopes, these rapidly flashing (“pulsing”) objects rapidly flashing (“pulsing”) objects were originally called were originally called pulsarspulsars..

PulsarsPulsars are just are just neutron starsneutron stars that that are easy to observe because the are easy to observe because the pulsing makes them stand out.pulsing makes them stand out.

The Neutron Star Mass The Neutron Star Mass LimitLimit Like white dwarfs, neutron stars Like white dwarfs, neutron stars

have a mass limit.have a mass limit. Believed to be 2.5-3.0 solar Believed to be 2.5-3.0 solar

masses (not known for sure)masses (not known for sure) If a neutron star is over this limit, If a neutron star is over this limit,

nothingnothing can stop its collapse. can stop its collapse. But what does it becomeBut what does it become??

Forming a Black HoleForming a Black Hole

ANY object that shrinks ANY object that shrinks enough will develop enough will develop surface gravity high surface gravity high enough to prevent enough to prevent everything from everything from escaping.escaping.

One example is a One example is a collapsing neutron star: collapsing neutron star: if it collapses enough if it collapses enough its surface gravity will its surface gravity will get intense enough to get intense enough to form a black hole.form a black hole.

Anatomy of a Black Anatomy of a Black HoleHole

Event Horizon

Ergosphere

Singularity

Anatomy of a Black Anatomy of a Black HoleHole The Event HorizonThe Event Horizon

– The point of no return – once you enter, you The point of no return – once you enter, you can never leave.can never leave.

– Inside all paths lead to the singularity.Inside all paths lead to the singularity. The ErgosphereThe Ergosphere

– Space itself getting dragged around the Space itself getting dragged around the black hole.black hole.

– Nothing can stay stationary within.Nothing can stay stationary within.– Once you enter, half your mass must go Once you enter, half your mass must go

into the event horizon so the other half can into the event horizon so the other half can escape.escape.

Anatomy of a Black Anatomy of a Black HoleHole Why don’t black holes suck in Why don’t black holes suck in

everything in the Universe?everything in the Universe?– Only dangerous if you are very closeOnly dangerous if you are very close

Black Holes Are Simple Black Holes Are Simple ObjectsObjects No Hair Theorem – Black holes have no No Hair Theorem – Black holes have no

hair.hair.– ““Hair” represents “details” – a black hole is Hair” represents “details” – a black hole is

described by only three quantities: mass, described by only three quantities: mass, electric charge, and rotation.electric charge, and rotation.

The Law of Cosmic Censorship – There The Law of Cosmic Censorship – There can be no naked singularities.can be no naked singularities.– Weird, universe-destroying things happen Weird, universe-destroying things happen

there! They must be “shielded” by an event there! They must be “shielded” by an event horizon.horizon.

The Schwarzschild The Schwarzschild RadiusRadius The size of a black hole’s event The size of a black hole’s event

horizon is related to its mass:horizon is related to its mass:

R = (3 km) M (in solar R = (3 km) M (in solar masses)masses)

So a 20 solar mass black hole has So a 20 solar mass black hole has a Schwarzschild Radius of 60 km.a Schwarzschild Radius of 60 km.

As a black hole eats, it gets As a black hole eats, it gets bigger!bigger!

The Accretion DiskThe Accretion Disk

Although we can’t Although we can’t directly observe directly observe the black hole, we the black hole, we can see the X-can see the X-rays created by rays created by superheated gas superheated gas flowing into the flowing into the hole in an hole in an accretion diskaccretion disk..

Observing Black Observing Black HolesHoles

No light can escape a No light can escape a black holeblack hole => Black holes can not be => Black holes can not be

observed directly.observed directly.If an invisible If an invisible compact object is compact object is part of a binary, part of a binary, we can estimate we can estimate its mass from the its mass from the orbital period orbital period and radial and radial velocity.velocity.Mass > 3 MMass > 3 Msunsun

=> Black => Black hole!hole!

Cygnus X-1Cygnus X-1

The first X-ray The first X-ray source discovered in source discovered in Cygnus was found to Cygnus was found to be a very compact be a very compact object (more than 3 object (more than 3 solar masses) in solar masses) in orbit around the orbit around the blue supergiant star blue supergiant star HDE 226868.HDE 226868.

First example of an First example of an X-ray source X-ray source believed to be a believed to be a black hole.black hole.

And Others…And Others…

But it was But it was certainly not the certainly not the last.last.

Many others have Many others have been found.been found.

AND the best AND the best evidence for real evidence for real black holes lurks black holes lurks at the centers of at the centers of galaxies!galaxies!

The Galactic NucleusThe Galactic Nucleus

The most The most mysterious part mysterious part of the galaxy.of the galaxy.

The very center The very center is a powerful is a powerful radio source radio source called Sagittarius called Sagittarius A*.A*.

Next TimeNext Time

Read Units 70 and 74Read Units 70 and 74

lecture 22

Documents

helium shell

helium fusion

helium flash

red giant branch

giant phase

asymptotic giant branch

horizontal branch stars

giant starsgiant stars